There are at least five general methods of preparing aliphatic polycarbonates. These are outlined in Polymer Reviews 9 "Chemistry and Physics of Polycarbonates," pp. 9-20. Aliphatic diols react with phosgene or bis-chlorocarbonic esters of aliphatic diols to produce polycarbonates. Cyclic carbonates of aliphatic dihydroxy compounds with larger than five-membered rings can be polymerized.
Transesterification of aliphatic dihydroxy compounds with carbonic acid diesters is a well-known and important method of preparing aliphatic polycarbonates. This method avoids the need to use phosgene, a toxic gas that is difficult to handle safely. Cyclic carbonates are usually produced when 1,2- or 1,3-diols are used, while polyalkylene carbonates are favored when four or more carbons separate the hydroxyl groups (See J. Am. Chem. Soc. 52 (1930) 314).
When dialkyl carbonates are used, transesterification will ordinarily not occur in the absence of a catalyst, even at elevated temperatures. Shimizu and Komiya reported (Japanese Kokai No. 1-252629) a synthesis of aliphatic polycarbonates by transesterification in the absence of a catalyst. According to the authors, the reaction works only with dimethyl carbonate and requires high temperatures (180.degree. C. to 250.degree. C.). Strongly basic catalysts, such as alkali metal alcoholates, are by far the catalysts most commonly used. Other transesterification catalysts described in the literature, typically bases and transition metal compounds, are outlined in U.S. Pat. Nos. 4,440,937, 3,426,042, and 3,663,569. These include, among other catalysts, oxides, hydroxides, alcoholates, carboxylates, and carbonates of sodium, potassium, aluminum, thallium, and lead, as well as various titanium compounds, metal chelates, and manganese salts.
Unfortunately, the transesterification catalysts known in the art suffer from a number of disadvantages. Catalyst residues present in polycarbonate polymers often adversely affect the thermal stability of thermoplastic resins by catalyzing depolymerization reactions ("unzipping") at the elevated temperatures used to process the polymers. Prepolymers made from isocyanates and polycarbonate polyols often gel prematurely if alkaline catalyst residues are present in the polycarbonate polyol. In addition, because alkali catalyzes depolymerization of the polycarbonates, polymers of high molecular weight (greater than about 1000) are often difficult to prepare and isolate. For these reasons, catalyst residues are usually removed--at great expense--before the polycarbonate product is used further. Onerous acid-washing and adsorption methods are commonly used to remove the alkaline catalyst residues from the polymers.
A method for preparing polyalkylene carbonate polymers that overcomes the problems with alkaline catalysts, and overcomes the need to remove a transesterification catalyst from a viscous polymer, is needed. Preferably, the method allows preparation of high molecular weight polyalkylene carbonate polymers under moderate conditions without the use of phosgene.